There's a fair amount of research that has looked at these issues. There seems to be consensus that that zing of passion and the breathlessness and butterflies in your stomach can only last about 18 months as the primary basis of relating. That's because it's partly based on anxiety and lack of knowledge -- anxiety about how the union is advancing, and lack of knowledge about the person and whether you even want to advance. For long-term relationships, liking and respect seem to be the keys. When couples who have been together happily for decades are asked why, the answer is almost always some variation on 'we're best friends.'
Passion doesn't need to go away, but over time it's not as shiny and new as it once was, and physiology just isn't the same when you're 65 as it was when you were 25.

It has been said that love is not so much a feeling as it is a way of being present.

They are, or can be, separate. Look at all the couples who are all over each other until they learn more about each other!

I wonder this myself... love to me has more than one meaning too. You can love someone, but not be in love with them. I love my family, I love my friends...
I think I have confused desire and passion for love and ended being fleeting. Then there's in love with passion and desire... that's the Best! I think you need a level of vulnerability to get all three and understanding.

I have never been sure.